The applicant for a patent of the present invention has proposed a vehicle-surroundings monitor apparatus described in Japanese Patent Application No. 2003-186603 applied earlier. This vehicle-surroundings monitor apparatus for a vehicle comprises a plurality of ultrasonic-wave sensors each having a processing function as sensors each used for measuring a distance to a body located in the vicinity of the vehicle and a control ECU for controlling the ultrasonic-wave sensors. The ultrasonic-wave sensors are connected to the control ECU by communication lines allowing data to be exchanged between the sensors and the ECU.
Each of the ultrasonic-wave sensors has a microphone, a transmission circuit, an IC and a regulator. The microphone is a component for transmitting an ultrasonic wave and receiving an ultrasonic wave reflected by a body existing in the vicinity of the vehicle. (An ultrasonic wave reflected by such a body is referred to hereafter as a reflected ultrasonic wave). The transmission circuit is a component for generating high-frequency and high-voltage pulses (having a typical frequency of several tens of kHz) for transmitting the ultrasonic wave from the microphone. The IC is a component for carrying out reception processing based on the reflected ultrasonic wave received by the microphone. The regulator is a component for converting a power-supply voltage received from the control ECU as a voltage for driving the ultrasonic-wave sensor into a constant voltage and also a component for supplying the constant voltage to the transmission circuit and the IC respectively.
When such an ultrasonic-wave sensor receives a command to find a distance to a body from the control ECU, the transmission circuit transmits an ultrasonic wave generated by the transmission circuit by way of the microphone and the microphone receives a reflected ultrasonic wave from a body existing in the vicinity of the vehicle. In addition, a period of time between the transmission of the ultrasonic wave and the reception of the reflected ultrasonic wave is measured. Then, a distance to the body is found based on the measured period of time. The distance is the distance between the microphone and the body.
In the ultrasonic-wave sensor described above, the regulator converts a power-supply voltage received from the control ECU into a constant voltage and supplies the constant voltage to the transmission circuit and the IC.
The inventors of the present invention have clearly found out that it is quite within the bounds of possibility that an ultrasonic-wave sensor employed in the vehicle-surroundings monitor apparatus having a processing function or, to be more specific, the IC employed in the ultrasonic-wave sensor does not operate normally as described below.
As described above, the ultrasonic-wave sensor having a processing function receives a power-supply voltage from the control ECU. The regulator converts this power-supply voltage into a constant voltage to be supplied to the transmission circuit and the IC so as to operate the ultrasonic-wave sensor having a processing function.
In order to transmit an ultrasonic wave from the microphone, the transmission circuit generates high-frequency high-voltage pulses. Then, the transmission circuit outputs the high-frequency high-voltage pulses to the microphone. Since the high-frequency high-voltage pulses are a voltage signal vibrating at a predetermined period, when the transmission circuit outputs the high-frequency high-voltage pulses, a voltage on the input side of the transmission circuit fluctuates due to an effect of the high-frequency high-voltage pulses. Thus, a voltage appearing on a wire supplying a power-supply voltage to the transmission circuit also varies.
An effect of the varying voltage propagates to the regulator through the wire connecting the transmission circuit to the regulator and also propagates to the IC through a wire connecting the regulator to the IC. Thus, a power-supply voltage supplied from the regulator to the IC also varies. As a result, in some cases, the IC receives a power-supply voltage lower than a power-supply voltage normally supplied to the IC.
That is to say, every time the transmission circuit generates a high-frequency high-voltage pulse and outputs the high-frequency high-voltage pulse to the microphone, a change in voltage results in the ultrasonic-wave sensor having a processing function. Thus, every time an ultrasonic wave is transmitted, a change in voltage is generated in a power-supply voltage supplied to the IC. As a result, a power-supply voltage necessary for operating the IC normally is no longer supplied to the IC. It is therefore quite within the bounds of possibility that the IC is reset, that is, internal circuits embedded in the IC do not operate.